Turbidity measurement plays an important role as an indicator of water quality. In the 13th edition of Standard Methods for the Examination of Water and Wastewater (1971), the American Public Health Association and the American Water Works Association accepted formazin as the primary standard for turbidity. Therefore, instrument specific alternative calibration standards such as styrene divinylbenzene microspheres, latex suspensions or metal oxide gels must be traced to formazin. The U.S. Environmental Protection Agency also recognises formazin as the primary turbidity standard in methods for the Chemical Examination of Water and Wastes.
Formazin is an aqueous suspension of an insoluble white polymer formed by the condensation reaction between hydrazine sulfate and hexamethylenetetramine. However, formazin degrades quickly when diluted to workable turbidity values in the range of 0-100 NTU using ultra-low turbidity water. However, formazin maintains its turbidity value for one year at a concentration of 4000 NTU, where residual reactants and reaction intermediates such as hexamethylenetetramine and sulfate compose the polymer's matrix. Dilutions with ultra-low turbidity water diluting the polymer matrix of formazin may facilitate the degradation of the polymer.
The preparation of formazin, as outlined in ISO Standard 7027:1999, requires a solution of 10 g/l hydrazine sulfate and a solution of 100 g/l hexamethylenetetramine. The two solutions are mixed in equal volumes. A white polymer suspension forms while the mixture stands for 24 hours at 25° C. Upon 1:10 dilution with ultra-low turbidity water, the suspension has a turbidity of 400 Formazin Nephelometric Units (FNU) or Nephelometric Turbidity Units (NTU). Lower turbidity standards require a dilution of the stock 4000 NTU solution with ultra-low turbidity water, and are reproducible within ±2% of reading. A 400 NTU formazin composition remains stable for one month when stored in darkness at 25° C., but working standards of lower turbidity degrade after one week or less. Below 40 NTU, the standards must be used immediately after preparation because degradation occurs in a day or less. More dilute standards degrade most quickly.
U.S. Pat. No. 5,777,011 discloses a stabilised formazin composition suitable as a primary turbidity standard. The composition can be prepared by diluting an aqueous formazin stock composition having a turbidity of greater than 400 NTU with a diluent containing hexamethylenetetramine (hexamine) and, optionally, a water-soluble sulfate salt such as sodium sulfate. The pH of the stabilised composition shall be in the range of about 7 to 11. Specifically, hexamine is used in a concentration range of 50 to 200 g/l, or another base is added to increase the pH of the composition.
Although U.S. Pat. No. 5,777,011 claims that the stabilised formazin solutions having a turbidity of less than 400 NTU will remain stable for a period of more than one year, it was found that slow degradation may still take place at low NTU values. In addition, use of hexamine solutions having a concentration of more than 50 g/l can result in significant refractive index changes leading to irregular turbidity readings.
Accordingly, there remains a need for further diluents and stabilised formazin solutions having a turbidity of less than 400 NTU useful as a primary calibration standard in turbidity measurements.